I am pretty sure Marianne's experiment is flawed

I don't really know the answer, but I have a few thoughts.

The post you linked to describes creating a central obstruction on the lens. This the reverse of what an aperture iris does. The iris blocks the outer area of the lens, leaving the center clear.

I'm familiar with central obstructions in telescopes. Most reflecting telescopes have a central obstruction (usually a mirror in the optical train). The larger the central obstruction, the lower the contrast of the scope. So, by adding a large central obstruction to her lens, Marianne is lowering image contrast that reaches the AF points. I don't know whether this contrast reduction alone is enough to be the cause for AF failure.

My (limited) understanding is that the size of a central objstruction does not change the f/ratio of a telescope. It is measured by the focal length divided by the dia. of the optics, regardless of the size of the central obstruction.

I believe you do have a point about the location of objstruction in the optical path. The lens iris is placed optical axis of the lens. However, Marianne may understand the concequences, of where her central obstruction is placed, better than I do.

--
Robin Casady
http://www.robincasady.com/Photo/index.html